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MAX4038 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX4038 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 19 page Low IBIAS, +1.4V/800nA, Rail-to-Rail Op Amps with +1.2V Buffered Reference ______________________________________________________________________________________ 11 Applications Information Power-Supply Considerations The MAX4036–MAX4039 operate from a single 1.4V (MAX4036/MAX4038) or 1.8V (MAX4037/MAX4039) to 3.6V supply. A high amplifier power-supply rejection ratio of 82dB and the excellent reference line regulation allow the devices to be powered directly from a decay- ing battery voltage, simplifying design and extending battery life. The MAX4036–MAX4039 are ideally suited for low-voltage battery-powered systems. The Typical Operating Characteristics show the changes in supply current and reference output as a function of supply voltage. Power-Up Settling Time The MAX4036–MAX4039 typically require 0.25ms to power-up. During this startup time, the output is inde- terminate. The application circuit should allow for this initial delay. See the Typical Operating Characteristics for amplifier and reference settling time curves. Driving Capacitive Loads: Op Amps The MAX4036–MAX4039 amplifier(s) require no output capacitor for stability, and are unity-gain stable for loads up to 5000pF. Applications that require greater capacitive-drive capability should use an isolation resistor between the output and the capacitive load (Figure 2). Note that this solution reduces the gain and output voltage swing because RISO forms a voltage- divider with the load resistor. Crossover Distortion The MAX4036–MAX4039 output stages are capable of sourcing and sinking currents with orders of magnitude greater than the stages’ quiescent current, which is less than 1µA. This ability to drive heavy loads with such a small quiescent current introduces crossover distortion as the output stage passes between sinking and sourcing. In the crossover regions, the output impedance of the MAX4036–MAX4039 increases sub- stantially, thereby changing the load-driving character- istics. The distortion can be greatly reduced by increasing the load resistance. For applications where low load resistance is required, bias the load such that the output current is always in one direction, to avoid crossover distortion. Reference Bypassing The MAX4037/MAX4039 reference requires no external capacitors. Using the MAX4036–MAX4039 as a Comparator Although optimized for use as an operational amplifier, the MAX4036–MAX4039 can be used as a rail-to-rail I/O comparator (Figures 3, 4). External hysteresis can be used to minimize the risk of output oscillation. The positive feedback circuit, shown in Figure 4, causes the input threshold to change when the output voltage changes state. Battery Monitoring Using the MAX4037/MAX4039 and Hysteresis The internal reference and low operating voltage of the MAX4037/MAX4039 make the devices ideal for battery- monitoring applications. Hysteresis can be set using resistors as shown in Figure 4, and the following design procedure: 1) Choose R3. The input bias current of IN_+ is under 100pA over temperature, so a current through R3 around 100nA maintains accuracy. The current through R3 at the trip point is VREF / R3, or 100nA for R3 = 12M Ω. 10MΩ is a good practical value. 2) Choose the hysteresis voltage (VHB), the voltage between the upper and lower thresholds. In this example, choose VHB = 50mV (see Figure 3). MAX4038 RISO RL CL RL RL + RISO AV = = 1 Figure 2. Using a Resistor to Isolate a Capacitive Load from the Op Amp VHB VOH VOL VTHR VTHF INPUT OUTPUT Figure 3. Hysteresis |
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